Synchronized Fluxional Motion and Cyclometalation of Ligands in Platinum(II) Complexes

Oscillation of the 2,9-dimethyl-1,10-phenanthroline (dmphen) ligand between nonequivalent exchanging sites in [Pt(Me)(dmphen)(P(o-tolyl)3)]+ and phosphane rotation around the Pt−P bond take place at the same rate. Thus, this cationic complex behaves as a molecular gear, exhibiting a fascinating synchronism between two otherwise independent fluxional motions. The process (ΔG333⧧ = 68.5 ± 0.2 kJ mol-1) was found to be unaffected by (i) the nature of various counteranions (X = PF6- 1, SbF6- 2, CF3SO3- 3, BF4- 4, BArf- 5), (ii) the polarity or the electron-donor properties of the solvent and, (iii) the addition of weak nucleophiles. Restricted phosphane rotation around the Pt−P bond impedes free dmphen oscillation in a 14-electron three-coordinate T-shaped intermediate, containing η1-coordinated dmphen, generated by easy Pt−N bond dissociation from [Pt(Me)(dmphen)(P(o-tolyl)3)]+. 1−5 undergo easy orthoplatination, leading to new [Pt(dmphen){CH2C6H4P(o-tolyl)2-κC,P}]X cyclometalated Pt(II) compounds (X = PF6- 1, SbF6- 2, CF3SO3- 3, BF4- 4, BArf- 5). The kinetics of the cyclometalation of 3 and 4 were followed in tetrachloroethane by both 1H NMR and spectrophotometric techniques (kobs = 1.7 × 10-4 s-1 at 333 K, ΔH⧧ = 59.3 ± 3 kJ mol-1, and ΔS⧧ = −141 ± 8 J K-1 mol-1). Ring opening of dmphen is again a prerequisite for C−H bond activation, which takes place through a multistep oxidative-addition reductive-elimination pathway. The molecular structure of cyclometalated 10 shows a butterfly shape with two o-tolyl rings projected above and below the coordination plane. Variable-temperature 1H NMR spectra revealed hindered rotation around the P−Cipso(o-tolyl) bonds at rather mild temperatures (ΔG333⧧ = 55.2 ± 0.4 kJ mol-1). Dmphen oscillation results very slowly and is dependent on the nature of the counteranions, of the solvents, and is strongly accelerated by the presence of weak nucleophiles that act as catalysts, according to an associative mode of activation.在阳离子配合物[Pt(Me)(dmphen)(P(邻甲苯基)₃)]⁺中，2,9-二甲基-1,10-菲咯啉（2,9-dimethyl-1,10-phenanthroline，dmphen）配体在非等价交换位点间的振荡，与三(邻甲苯基)膦（P(o-tolyl)₃）围绕Pt−P键的旋转速率完全一致。因此，该阳离子配合物可作为分子齿轮（molecular gear），展现出两种原本独立的流变性运动（fluxional motion）之间的精妙同步性。该过程的活化吉布斯自由能（ΔG₃₃₃⧧ = 68.5 ± 0.2 kJ·mol⁻¹）不受以下因素影响：(i) 各类反阴离子（counteranion）的性质（X = PF₆⁻ 1、SbF₆⁻ 2、CF₃SO₃⁻ 3、BF₄⁻ 4、BArf⁻ 5）；(ii) 溶剂的极性与给电子性能；以及(iii) 弱亲核试剂的添加。从[Pt(Me)(dmphen)(P(邻甲苯基)₃)]⁺中易于发生的Pt−N键解离，会生成含η¹配位dmphen的14电子三坐标T形中间体，该中间体中膦配体围绕Pt−P键的受限旋转会阻碍dmphen的自由振荡。 化合物1~5易发生邻位铂化（orthoplatination），生成新型环金属化Pt(II)化合物[Pt(dmphen){CH₂C₆H₄P(邻甲苯基)₂-κC,P}]X（X = PF₆⁻ 1、SbF₆⁻ 2、CF₃SO₃⁻ 3、BF₄⁻ 4、BArf⁻ 5）。通过¹H核磁共振（¹H NMR）与分光光度法（spectrophotometric technique），可追踪化合物3和4在四氯乙烷中的环金属化动力学：333 K下观测速率常数kobs = 1.7 × 10⁻⁴ s⁻¹，活化焓（ΔH⧧ = 59.3 ± 3 kJ·mol⁻¹）与活化熵（ΔS⧧ = −141 ± 8 J·K⁻¹·mol⁻¹）均被测定。dmphen的环张开再次成为C−H键活化的先决条件，该活化过程通过多步氧化加成-还原消除（oxidative-addition reductive-elimination）路径进行。 环金属化产物10的分子结构呈蝶形，两个邻甲苯基环分别位于配位平面的上下两侧。变温¹H核磁共振光谱显示，在较为温和的温度下，P−邻甲苯基ipso碳键（P−Cipso(o-tolyl)）的旋转存在位阻，其活化吉布斯自由能为ΔG₃₃₃⧧ = 55.2 ± 0.4 kJ·mol⁻¹。dmphen的振荡速率极慢，且受反阴离子性质与溶剂种类影响；同时，以缔合活化模式运作的弱亲核试剂可作为催化剂，显著加速该振荡过程。